pgsql: deflist_to_tuplestore dumped core on an option with no value.

deflist_to_tuplestore dumped core on an option with no value.

Make it return NULL for the option_value, instead.

Per report from Frank van Vugt.  Back-patch to 8.4 where this code was
added.

Branch
------
REL9_1_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/663bee0d5c1e3ff2380377c7894499ae91eabd87

Modified Files
--------------
src/backend/foreign/foreign.c |   13 +++++++++++--
1 files changed, 11 insertions(+), 2 deletions(-)